This invention relates to a rotary drill bit used in drilling for oil, iron ore and coal or in drilling operations for geothermal power generation.
In a rotary drill bit of the type described, a cutter is fitted onto the journal pin of a leg extending from the bit body. It is essential that means be provided for retaining the cutter on the journal pin so that the cutter will not undergo displacement axially of the journal, namely along the axis of cutter rotation. An example of such retention means is disclosed in the specification of U.S. Pat. No. 4,444,518. In one arrangement, a garter spring is brought into wrapping engagement with a plurality of segmented rings to engage the segmented rings with a groove in the cutter and a groove in the journal pin when these two grooves are registered with each other. In an alternative arrangement, the garter spring is replaced by leaf springs secured to the outer face of respective ones of he segmented rings to engage the segmented rings with the aforementioned grooves in a similar manner.
The specifications of U.S. Pat. Nos. 4,236,764 and 4,511,008 disclose arrangements in which only a snap ring is accommodated so as to bridge the cutter groove and journal pin groove to retain the cutter in the axial direction.
A number of problems are encountered in the prior art mentioned above. Specifically, in the arrangement where the garter spring is used, spring durability declines due to the extremely harsh conditions under which the drill bit is used. In the arrangement that relies upon the leaf springs secured to the segmented rings, the leaf springs and segmented rings tend to separate from each other owing to a concentration of stress produced at the spring-ring joints by repetitive application of lateral force. In addition, the segmented rings are fitted into the grooves so as not to readily move in the axial direction in order to prevent the cutter from undergoing conical motion. Consequently, fragments of metal are produced (a phenomenon referred to as "spalding") due to friction among the component parts during operation of the drill bit, and the metal fragments cause an O-ring on the journal pin to become flawed and scored. The end result is that the sealing capability of the O-ring diminishes in a short period of time. Even if the drawback of spalding can be alleviated to some extent, the segmented rings undergo almost no movement axially of the cutter. In consequence, an axially directed breaking force applied to the segmented rings acts upon the rings directly and without mitigation. The segmented rings therefore are likely to break or become damaged. This tendency makes it difficult to avoid a rapid end to the service life of the bit.
In the arrangement using the snap ring as the axial retention means, the effects of the strong lateral force and axially directed breaking force mentioned above appear and make it difficult to prolong bit lifetime.